How do you say the total amount of time that has been spent in a job position?

Tenure

In one of its meanings:

a. The act, fact, manner, or condition of holding something in one's possession, as real estate or an office; occupation.
b. A period during which something is held.

(American Heritage Dictionary)

Examples of use:

Average job tenure has remained unchanged, subject to cyclical variation, for the last 25 years. The average duration of a job in progress is around five years. (source)
Why are Wages Upward Sloping with Tenure? … We might expect the hazard rate from jobs to decrease nonmonotonically with tenure. (source)

You are probably looking for:

Seniority:

the advantage that you get by working for a company for a long time:

• In future, promotion will be based on merit not seniority.

(Cambridge Dictionary)